Data processing apparatus and method

ABSTRACT

A data processing apparatus maps input symbols to be communicated onto a predetermined number of sub-carrier signals of an Orthogonal Frequency Division Multiplexed (OFDM) symbol. The data processor includes an interleaver memory which reads-in the predetermined number of data symbols for mapping onto the OFDM sub-carrier signals. The interleaver memory reads-out the data symbols on to the OFDM sub-carriers to effect the mapping, the read-out being in a different order than the read-in, the order being determined from a set of addresses, with the effect that the data symbols are interleaved on to the sub-carrier signals. The set of addresses are generated from an address generator which comprises a linear feedback shift register and a permutation circuit.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Continuation-In-Part application of thefollowing U.S. Applications:

-   U.S. application Ser. No. 12/256,095, filed Oct. 22, 2008, and    claims priority to United Kingdom Applications Nos. 0721269.9, filed    Oct. 30, 2007; 0722645.9, filed Nov. 19, 2007; and 0722728.3, filed    Nov. 20, 2007;-   U.S. application Ser. No. 12/257,999, filed Oct. 24, 2008, and    claims priority to United Kingdom Applications Nos. 0721269.9, filed    Oct. 30, 2007; 0722645.9, filed Nov. 19, 2007; and 0722728.3, filed    Nov. 20, 2007;-   U.S. application Ser. No. 12/249,306, filed Oct. 10, 2008, and    claims priority to United Kingdom Applications Nos. 0721271.5, filed    Oct. 30, 2007; 0721269.9, filed Oct. 30, 2007; 0722645.9, filed Nov.    19, 2007; and 0722728.3, filed Nov. 20, 2007;-   U.S. application Ser. No. 12/257,010, filed Oct. 23, 2008, and    claims priority to United Kingdom Applications Nos. 0721269.9, filed    Oct. 30, 2007; 0722645.9, filed Nov. 19, 2007; and 0722728.3, filed    Nov. 20, 2007;-   U.S. Application No. 13/235,128, filed Sep. 16, 2011, which is a    divisional application of U.S. application Ser. No. 12/051,541,    filed Mar. 19, 2008, which is a continuation of U.S. application    Ser. No. 10/806,524, filed on Mar. 23, 2004, now U.S. Pat. No.    7,426,240, which is claims priority to EP 03290754.5, filed Mar. 25,    2003.-   U.S. application Ser. No. 12/249,276, filed Oct. 10, 2008, and    claims priority to United Kingdom Applications Nos. 0721272.3, filed    Oct. 30, 2007;-   U.S. application Ser. No. 12/249,294, filed Oct. 10, 2008, and    claims priority to United Kingdom Applications Nos. 0721271.5, filed    Oct. 30, 2007; 0721269.9, filed Oct. 30, 2007; 0722645.9, filed Nov.    19, 2007; and 0722728.3, filed Nov. 20, 2007; and-   U.S. application Ser. No. 12/256,746, filed Oct. 23, 2008, and    claims priority to United Kingdom Applications Nos. 0721269.9, filed    Oct. 30, 2007; 0721270.7, filed Oct. 30, 2007; 0722645.9, filed Nov.    19, 2007; and 0722728.3, filed Nov. 20, 2007. The entire contents of    the above-identified applications are incorporated herein by    reference.

FIELD OF INVENTION

The present invention relates to data processing apparatus operable tomap input symbols onto sub-carrier signals of Orthogonal FrequencyDivision Multiplexed (OFDM) symbols.

The present invention also relates to data processing apparatus operableto map symbols received from a predetermined number of sub-carriersignals of OFDM symbols into an output symbol stream.

Embodiments of the present invention can provide an OFDMtransmitter/receiver.

BACKGROUND OF THE INVENTION

The Digital Video Broadcasting-Terrestrial standard (DVB-T) utilisesOrthogonal Frequency Division Multiplexing (OFDM) to communicate datarepresenting video images and sound to receivers via a broadcast radiocommunications signal. There are known to be two modes for the DVB-Tstandard which are known as the 2k and the 8k mode. The 2k mode provides2048 sub-carriers whereas the 8k mode provides 8192 sub-carriers.Similarly for the Digital Video Broadcasting-Handheld standard (DVB-H) a4k mode has been provided, in which the number of sub-carriers is 4096.

Error correction coding schemes such as Low Density ParityCheck/Bose-Chaudhuri-Hocquengham (LDPC/BCH) coding, which have beenproposed for DVB-T2 perform better when noise and degradation of thesymbol values resulting from communication is un-correlated. Terrestrialbroadcast channels may suffer from correlated fading in both the timeand the frequency domains. As such, by separating encoded symbols on todifferent sub-carrier signals of the OFDM symbol by as much as possible,the performance of error correction coding schemes can be increased.Accordingly, in order to improve the integrity of data communicatedusing DVB-T or DVB-H, a symbol interleaver is provided in order tointerleave input data symbols as these symbols are mapped onto thesub-carrier signals of an OFDM symbol. Such a symbol interleavercomprises an interleaver memory and an address generator. Theinterleaver is arranged to read-into the interleaver memory the datasymbols for mapping onto the OFDM sub-carrier signals, and to read-outof the memory the data symbols for the OFDM sub-carriers, the read-outbeing in a different order than the read-in, the order being determinedfrom a set of addresses, which are generated by the address generator.For the 2k mode and the 8k mode an arrangement has been disclosed in theDVB-T standard for generating the addresses to effect the mapping.Likewise for the 4k mode of DVB-H standard, an arrangement forgenerating addresses for the mapping has been provided and an addressgenerator for implementing this mapping is disclosed in European Patentapplication 04251667.4. The address generator comprises a linear feedback shift register which is operable to generate a pseudo random bitsequence and a permutation circuit. The permutation circuit permutes theorder of the content of the linear feed back shift register in order togenerate an address. The address provides an indication of a memorylocation of the interleaver memory for writing the input data symbolinto or reading the input data symbol out from the interleaver memoryfor mapping onto one of the sub-carrier signal of the OFDM symbol.Similarly, an address generator in the receiver is arranged to generateaddresses of the interleaver memory for writing the received datasymbols into or reading the data symbols out from the interleaver memoryto form an output data stream.

In accordance with a further development of the Digital VideoBroadcasting-Terrestrial standard, known as DVB-T2, there is a desire toimprove the communication of data, and more particularly to provide animproved arrangement for interleaving the data symbols onto thesub-carrier signals of OFDM symbols.

SUMMARY OF INVENTION

According to an aspect of the present invention there is provided a dataprocessing apparatus operable to map input data symbols to becommunicated onto a predetermined number of sub-carrier signals of anOrthogonal Frequency Division Multiplexed (OFDM) symbols. The dataprocessing apparatus comprises an interleaver operable to read-into amemory the predetermined number of data symbols for mapping onto theOFDM sub-carrier signals, and to read-out of the memory the data symbolsfor the OFDM sub-carriers to effect the mapping, the read-out being in adifferent order than the read-in, the order being determined from a setof addresses, with the effect that the data symbols are interleaved onthe sub-carrier signals. The data processing apparatus includes anaddress generator operable to generate the set of addresses, an addressbeing generated for each of the input data symbols for mapping the inputdata symbols onto the sub-carrier signals. The address generatorcomprises a linear feedback shift register including a predeterminednumber of register stages, which are operable to generate apseudo-random bit sequence in accordance with a generator polynomial, apermutation circuit arranged to receive the content of the shiftregister stages and to permute the order of the bits present in theregister stages in accordance with a permutation code to form an addressof one of the OFDM sub-carriers, and a control unit operable incombination with an address check circuit to re-generate an address whena generated address exceeds a predetermined maximum valid address. Thepredetermined maximum valid address is approximately eight thousand, thelinear feedback shift register has twelve register stages with agenerator polynomial for the linear feedback shift register ofR′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6], and thepermutation order forms, with an additional bit, a thirteen bit address.The data processing apparatus is characterised in that the permutationcircuit is arranged to change the permutation code, which permutes theorder of the bits of the register stages to form the set of addressesfrom one OFDM symbol to another.

Embodiments of the present invention can provide a data processingapparatus operable as a symbol interleaver for mapping data symbols tobe communicated on an OFDM symbol, having substantially eight thousandsub-carrier signals, which can provide an improvement in the integrityof the communicated data. The improvement is provided as a result of achange of the permutation code, which is used to change the order of thebits in the feed back shift register, from one OFDM symbol to another.For example, the permutation code used may be one of a sequence ofdifferent permutation codes which is cycled through, for each of aplurality of OFDM symbols. As a result, an improvement is provided inreducing a possibility that successive or data bits which are close inorder in an input data stream are mapped onto the same sub-carrier of anOFDM symbol, so that the error correction encoding can work moreefficiently.

In one embodiment the number of sub-carrier signals maybe a valuesubstantially between six thousand and eight thousand one hundred andninety two. Furthermore, the OFDM symbol may include pilot sub-carriers,which are arranged to carry known symbols, and the predetermined maximumvalid address may depend on a number of the pilot sub-carrier symbolspresent in the OFDM symbol. As such the 8k mode can be provided with anefficient symbol interleaver, for example for a DVB standard, such asDVB-T2, DVB-T or DVB-H.

In one example the sequence of different permutation codes forms thethirteen bit address R_(i)[n] for the i-th data symbol from the bitpresent in the n-th register stage R′_(i)[n] in accordance with thepermutation code defined by the table:

R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 5 113 0 10 8 6 9 2 4 1 7

Although the sequence of permutation codes can include any number ofpermutation codes, in one example there are two permutation codes. Inone example, the two permutation codes are:

R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 5 113 0 10 8 6 9 2 4 1 7

and

R′_(i) bit positions 11 10 9 8 7 6 5 4 3 2 1 0 R_(i) bit positions 8 107 6 0 5 2 1 3 9 4 11

For example, the approximately eight thousand sub-carriers may beprovided as one of a plurality of operating modes, the approximatelyeight thousand sub-carriers providing half or less than half a maximumnumber of sub-carriers in the OFDM symbols of any of the operatingmodes. The input data symbols may be formed into or regarded as firstsets of input data symbols for mapping onto first OFDM symbols andsecond sets of input data symbols for mapping onto second OFDM symbols.The data processing apparatus may be operable to interleave the inputdata symbols from both first and second sets in accordance with an oddinterleaving process. The odd interleaving process includes writing thefirst sets of input data symbols into a first part of the interleavermemory in accordance with a sequential order of the first sets of inputdata symbols, reading out the first sets of input data symbols from thefirst part of the interleaver memory on to the sub-carrier signals ofthe first OFDM symbols in accordance with an order defined by one of thepermutation codes of the sequence, writing the second set of input datasymbols into a second part of the interleaver memory in accordance witha sequential order of the second sets of input data symbols, and readingout the second sets of input data symbols from the second part of theinterleaver memory on to the sub-carrier signals of the second OFDMsymbols in accordance with an order defined by another of thepermutation codes of the sequence.

The first OFDM symbols may be odd OFDM symbols, and the second OFDMsymbols may be even OFDM symbols.

In some conventional OFDM transmitters and receivers, which operate inaccordance with the 2k or 8k modes for DVB-T and the 4k mode for DVB-H,two symbol interleaving processes are used in the transmitter and thereceiver; one for even OFMD symbols and one for odd OFMD symbols.However, analysis has shown that the interleaving schemes designed forthe 2k and 8k symbol interleavers for DVB-T and the 4k symbolinterleaver for DVB-H work better for odd symbols than for even symbols.Embodiments of the present invention are arranged so that only the oddsymbol interleaving process is used unless the transmitter/receiver isin the mode with the maximum number of sub-carriers. Therefore, when thenumber data symbols which can be carried by the sub-carriers of an OFDMsymbol in one of the plurality of operating modes is less than half ofthe number of data symbols, which can be carried in an operating modewhich proves the most number of data bearing sub-carrier signals perOFDM symbol, then an interleaver of the transmitter and the receiver ofthe OFDM symbols is arranged to interleaver the data symbols of both thefirst and second sets using the odd interleaving process. Since theinterleaver is interleaving the data symbols of both the first andsecond sets of data symbols onto the OFDM symbols using the oddinterleaving process, the interleaver uses different parts of theinterleaver memory to write in and read out the data symbols. Thus,compared with the example in which the interleaver is using the oddinterleaving process and the even interleaving process to interleave thefirst and second sets of data symbols onto successive first and secondOFDM symbols, which utilises the available memory, the amount of memorycapacity used is twice the number of data symbols which can be carriedby an OFDM symbol for the odd only interleaving. This is compared with amemory requirement of one times the number of data symbols, which can becarried in an OFDM symbol in the mode with the most number of datasymbols per OFDM symbol using both the odd and even interleavingprocesses. However, the number of sub-carriers per OFDM symbol for thismaximum operating mode is twice the capacity of the next largest numberof sub-carriers per OFDM symbol for any other operating mode with thenext largest number of sub-carriers per OFDM symbol.

According to some examples therefore, a minimum size of the interleavermemory can be provided in accordance with the maximum number of inputdata symbols which can be carried on the sub-carriers of the OFDMsymbols which are available to carry the input data symbols in any ofthe operating modes.

In some embodiments the operating mode which provides the maximum numberof sub-carriers per OFDM symbol is a 32k mode. The other modes mayinclude one or more of 1k, 2k, 4k, 8k and 16k modes. Thus, as will beappreciated from the above explanation, in the 32k mode the odd and eveninterleaving processes are used to interleave the data symbols, so thatthe size of the interleaver memory can be just enough to account for 32kdata symbols. However, for the 16k mode and any of the other modes, thenthe odd interleaving process only is used, so that with the 16k mode anequivalent memory size of 32k symbols is required, with the 4k mode anequivalent memory size of 8k symbols is required, and with the 2k modean equivalent memory size of 4k symbols is required.

Various aspects and features of the present invention are defined in theappended claims. Further aspects of the present invention include amethod of mapping input symbols to be communicated onto a predeterminednumber of sub-carrier signals of an Orthogonal Frequency DivisionMultiplexed (OFDM) symbol, as well as a transmitter.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, wherein likeparts are provided with corresponding reference numerals, and in which:

FIG. 1 is a schematic block diagram of a Coded OFDM transmitter whichmay be used, for example, with the DVB-T2 standard;

FIG. 2 is a schematic block diagram of parts of the transmitter shown inFIG. 1 in which a symbol mapper and a frame builder illustrate theoperation of an interleaver;

FIG. 3 is a schematic block diagram of the symbol interleaver shown in

FIG. 2;

FIG. 4 is a schematic block diagram of an interleaver memory shown inFIG. 3 and the corresponding symbol de-interleaver in the receiver;

FIG. 5 is a schematic block diagram of an address generator shown inFIG. 3 for the 8k mode;

FIG. 6 is a schematic block diagram of a Coded OFDM receiver which maybe used, for example, with the DVB-T2 standard;

FIG. 7 is a schematic block diagram of a symbol de-interleaver whichappears in FIG. 6;

FIG. 8( a) is diagram illustrating results for an interleaver for evenOFDM symbols and FIG. 8( b) is a diagram illustrating results for oddOFDM symbols; FIGS. 8( a) and 8(b) show plots of the distance at theinterleaver output of sub-carriers that were adjacent at the interleaverinput;

FIG. 9 provides a schematic block diagram of the symbol interleavershown in FIG. 3, illustrating an operating mode in which interleaving isperformed in accordance with an odd interleaving mode only; and

FIG. 10 provides a schematic block diagram of the symbol de-interleavershown in FIG. 7, illustrating the operating mode in which interleavingis performed in accordance with the odd interleaving mode only.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following description is provided to illustrate the operation of asymbol interleaver in accordance with the present technique, although itwill be appreciated that the symbol interleaver can be used with othermodes, other DVB standards and other OFDM systems.

FIG. 1 provides an example block diagram of a Coded OFDM transmitterwhich may be used for example to transmit video images and audio signalsin accordance with the DVB-T2 standard. In FIG. 1 a program sourcegenerates data to be transmitted by the Coded Orthogonal FrequencyDivision Multiplexing (COFDM) transmitter. A video coder 2, and audiocoder 4 and a data coder 6 generate video, audio and other data to betransmitted which are fed to a program multiplexer 10. The output of theprogram multiplexer 10 forms a multiplexed stream with other informationrequired to communicate the video, audio and other data. The multiplexer10 provides a stream on a connecting channel 12. There may be many suchmultiplexed streams which are fed into different branches A, B etc. Forsimplicity, only branch A will be described.

As shown in FIG. 1 a COFDM transmitter receives the stream at amultiplexer adaptation and energy dispersal block 22. The multiplexeradaptation and energy dispersal block 22 randomises the data and feedsthe appropriate data to a forward error correction encoder 24 whichperforms error correction encoding of the stream. A bit interleaver 26is provided to interleave the encoded data bits which for the example ofDVB-T2 is the LDCP/BCH encoder output. The output from the bitinterleaver 26 is fed to a bit into constellation mapper 28, which mapsgroups of bits onto a constellation point, which is to be used forconveying the encoded data bits. The outputs from the bit intoconstellation mapper 28 are constellation point labels that representreal and imaginary components. The constellation point labels representdata symbols formed from two or more bits depending on the modulationscheme used. These will be referred to as data cells. These data cellsare passed through a time-interleaver 30 whose effect is to interleaverdata cells resulting from multiple LDPC code words.

The data cells are received by a frame builder 32, with data cellsproduced by branch B etc in FIG. 1, via other channels 31. The framebuilder 32 then forms many data cells into sequences to be conveyed onCOFDM symbols, where a COFDM symbol comprises a number of data cells,each data cell being mapped onto one of the sub-carriers. The number ofsub-carriers will depend on the mode of operation of the system, whichmay include one of 1k, 2k, 4k, 8k, 16k or 32k, each of which provides adifferent number of sub-carriers according, for example to the followingtable:

Number of Sub-carriers Adapted from DVB-T/H Mode Sub-carriers  1k 756 2k 1512  4k 3024  8k 6048 16k 12096 32k 24192

Thus in one example, the number of sub-carriers for the 8k mode is sixthousand and forty eight. For the DVB-T2 system, the number ofsub-carriers per OFDM symbol can vary depending upon the number of pilotand other reserved carriers. Thus, in DVB-T2, unlike in DVB-T, thenumber of sub-carriers for carrying data is not fixed. Broadcasters canselect one of the operating modes from 1k, 2k, 4k, 8k, 16k, 32k eachproviding a range of sub-carriers for data per OFDM symbol, the maximumavailable for each of these modes being 1024, 2048, 4096, 8192, 16384,32768 respectively. In DVB-T2 a physical layer frame is composed of manyOFDM symbols. Typically the frame starts with one or more preamble or P2OFDM symbols, which are then followed by a number payload carrying OFDMsymbols. The end of the physical layer frame is marked by a frameclosing symbols. For each operating mode, the number of sub-carriers maybe different for each type of symbol. Furthermore, this may vary foreach according to whether bandwidth extension is selected, whether tonereservation is enabled and according to which pilot sub-carrier patternhas been selected. As such a generalisation to a specific number ofsub-carriers per OFDM symbol is difficult. However, the frequencyinterleaver for each mode can interleave any symbol whose number ofsub-carriers is smaller than or the same as the maximum available numberof sub-carriers for the given mode. For example, in the 1k mode, theinterleaver would work for symbols with the number of sub-carriers beingless than or equal to 1024 and for 16k mode, with the number ofsub-carriers being less than or equal to 16384.

The sequence of data cells to be carried in each COFDM symbol is thenpassed to the symbol interleaver 33. The COFDM symbol is then generatedby a COFDM symbol builder block 37 which introduces pilot andsynchronising signals fed from a pilot and embedded signal former 36. AnOFDM modulator 38 then forms the OFDM symbol in the time domain which isfed to a guard insertion processor 40 for generating a guard intervalbetween symbols, and then to a digital to analogue convertor 42 andfinally to an RF amplifier within an RF front end 44 for eventualbroadcast by the COFDM transmitter from an antenna 46.

As explained above, the present invention provides a facility forproviding a quasi-optimal mapping of the data symbols onto the OFDMsub-carrier signals. According to the example technique the symbolinterleaver is provided to effect the optimal mapping of input datasymbols onto COFDM sub-carrier signals in accordance with a permutationcode and generator polynomial, which has been verified by simulationanalysis.

As shown in FIG. 2 a more detailed example illustration of the bit tosymbol constellation mapper 28 and the frame builder 32 is provided toillustrate an example embodiment of the present technique. Data bitsreceived from the bit interleaver 26 via a channel 62 are grouped intosets of bits to be mapped onto a data cell, in accordance with a numberof bits per symbol provided by the modulation scheme. The groups ofbits, which forms a data word, are fed in parallel via data channels 64the a mapping processor 66. The mapping processor 66 then selects one ofthe data symbols, in accordance with a pre-assigned mapping. Theconstellation point, is represented by a real and an imaginary componentthat is provided to the output channel 29 as one of a set of inputs tothe frame builder 32.

The frame builder 32 receives the data cells from the bit toconstellation mapper 28 through channel 29, together with data cellsfrom the other channels 31. After building a frame of many COFDM cellsequences, the cells of each COFDM symbol are then written into aninterleaver memory 100 and read out of the interleaver memory 100 inaccordance with write addresses and read addresses generated by anaddress generator 102. According to the write-in and read-out order,interleaving of the data cells is achieved, by generating appropriateaddresses. The operation of the address generator 102 and theinterleaver memory 100 will be described in more detail shortly withreference to FIGS. 3, 4 and 5. The interleaved data cells are thencombined with pilot and synchronisation symbols received from the pilotand embedded signalling former 36 into an OFDM symbol builder 37, toform the COFDM symbol, which is fed to the OFDM modulator 38 asexplained above.

Interleaver

FIG. 3 provides an example of parts of the symbol interleaver 33, whichillustrates the present technique for interleaving symbols. In FIG. 3the input data cells from the frame builder 32 are written into theinterleaver memory 100. The data cells are written into the interleavermemory 100 according to a write address fed from the address generator102 on channel 104, and read out from the interleaver memory 100according to a read address fed from the address generator 102 on achannel 106. The address generator 102 generates the write address andthe read address as explained below, depending on whether the COFDMsymbol is odd or even, which is identified from a signal fed from achannel 108, and depending on a selected mode, which is identified froma signal fed from a channel 110. As explained, the mode can be one of a1k mode, 2k mode, 4k mode, 8k mode, 16k mode or a 32k mode. As explainedbelow, the write address and the read address are generated differentlyfor odd and even symbols as explained with reference to FIG. 4, whichprovides an example implementation of the interleaver memory 100.

In the example shown in FIG. 4, the interleaver memory is shown tocomprise an upper part 100 illustrating the operation of the interleavermemory in the transmitter and a lower part 340, which illustrates theoperation of the de-interleaver memory in the receiver. The interleaver100 and the de-interleaver 340 are shown together in FIG. 4 in order tofacilitate understanding of their operation. As shown in FIG. 4 arepresentation of the communication between the interleaver 100 and thede-interleaver 340 via other devices and via a transmission channel hasbeen simplified and represented as a section 140 between the interleaver100 and the de-interleaver 340. The operation of the interleaver 100 isdescribed in the following paragraphs:

Although FIG. 4 provides an illustration of only four input data cellsonto an example of four sub-carrier signals of a COFDM symbol, it willbe appreciated that the technique illustrated in FIG. 4 can be extendedto a larger number of sub-carriers such as 756 for the 1k mode 1512 forthe 2k mode, 3024 for the 4k mode and 6048 for the 8k mode, 12096 forthe 16k mode and 24192 for the 32k mode.

The input and output addressing of the interleaver memory 100 shown inFIG. 4 is shown for odd and even symbols. For an even COFDM symbol thedata cells are taken from the input channel and written into theinterleaver memory 124.1 in accordance with a sequence of addresses 120generated for each COFDM symbol by the address generator 102. The writeaddresses are applied for the even symbol so that as illustratedinterleaving is effected by the shuffling of the write-in addresses.Therefore, for each interleaved symbol y(h(q))=y′(q).

For odd symbols the same interleaver memory 124.2 is used. However, asshown in FIG. 4 for the odd symbol the write-in order 132 is in the sameaddress sequence used to read out the previous even symbol 126. Thisfeature allows the odd and even symbol interleaver implementations toonly use one interleaver memory 100 provided the read-out operation fora given address is performed before the write-in operation. The datacells written into the interleaver memory 124.2 during odd symbols arethen read out in a sequence 134 generated by the address generator 102for the next even COFDM symbol and so on. Thus only one address isgenerated per symbol, with the read-in and write-out for the odd/evenCOFDM symbol being performed contemporaneously.

In summary, as represented in FIG. 4, once the set of addresses H(q) hasbeen calculated for all active sub-carriers, the input vectorY′=(y_(0′), y_(1′), y_(2′), . . . y_(Nmax-1′)) is processed to producethe interleaved vector Y=(y₀, y₁, y₂, . . . y_(Nmax-1)) defined by:

y_(H(q))=y′_(q) for even symbols for q=0, . . . , N_(max)−1

y_(q)=y′_(H(q)) for odd symbols for q=0, . . . , N_(max)−1

In other words, for even OFDM symbols the input words are written in apermutated way into a memory and read back in a sequential way, whereasfor odd symbols, they are written sequentially and read back permutated.In the above case, the permutation H(q) is defined by the followingtable:

TABLE 1 permutation for simple case where Nmax = 4 q 0 1 2 3 H(q) 1 3 02

As shown in FIG. 4, the de-interleaver 340 operates to reverse theinterleaving applied by the interleaver 100, by applying the same set ofaddresses as generated by an equivalent address generator, but applyingthe write-in and read-out addresses in reverse using memories 341.1 and341.2. As such, for even symbols, the write-in addresses 342 are insequential order, whereas the read out address 344 are provided by theaddress generator. Correspondingly, for the odd symbols, the write-inorder 346 is determined from the set of addresses generated by theaddress generator, whereas read out 348 is in sequential order.

Address Generation for the 8k Mode

A schematic block diagram of the algorithm used to generate thepermutation function H(q) is represented in FIG. 5 for the 8K mode. InFIG. 5 a linear feed back shift register is formed by twelve shiftregister stages 200, in order to generate an address between 0 and 8191,and an xor-gate 202 which is connected to the stages of the shiftregister 200 in accordance with a generator polynomial. Therefore, inaccordance with the content of the shift register 200 a next bit of theshift register is provided from the output of the xor-gate 202 by xoringthe content of shift register R[0] and register stage R[1]. According tothe generator polynomial a pseudo random bit sequence is generated fromthe content of the shift register 200. However, in order to generate anaddress for the 8k mode as illustrated, a permutation circuit 210 isprovided which effectively permutes the order of the bits within theshift register 200 from an order R′_(i)[n] to an order R_(i)[n] at theoutput of the permutation circuit 210. Twelve bits from the output ofthe permutation circuit 210 are then fed on a connecting channel 212 towhich is added a most significant bit via a channel 214 which isprovided by a toggle circuit 218. A thirteen bit address is thereforegenerated on channel 212. However, in order to ensure the authenticityof an address, an address check circuit 216 analyses the generatedaddress to determine whether it exceeds the maximum number ofsub-carrier signals. If it does then a control signal is generated andfed via a connecting channel 220 to a control unit 224. If the generatedaddress exceeds the maximum number of carrier signals then this addressis rejected and a new address regenerated for the particular symbol.

In summary an (N_(r)−1) bit word R′_(i) is defined, with N_(r)=log₂M_(max), where M_(max)=8 192 in the 8k mode, using a LFSR (LinearFeedback Shift Register).

The polynomials used to generate this sequence are as follows:

8k mode: R′_(i)[11]=R′_(i-1)[0]⊕R′_(i-1)[1]⊕R′_(i-1)[4]⊕R′_(i-1)[6]

where i varies from 0 to M_(max)−1

Once one R′_(i), word has been generated, it goes through a permutationto produce another (N_(r)−1) bit word called R_(i). R_(i) is derivedfrom R′_(i) by the bit permutations given in the table below.

TABLE Bit permutation for the 8k mode R′_(i) bit positions 11 10 9 8 7 65 4 3 2 1 0 R_(i) bit positions 5 11 3 0 10 8 6 9 2 4 1 7

As an example, for the permutation code above this means that for mode8k, the bit number 11 of R′_(i) is sent in bit position number 5 ofR_(i).

The address H(q) is then derived from R_(i) through the followingequation:

${H(q)} = {{( {i\mspace{14mu}{mod}\mspace{11mu} 2} ) \cdot 2^{N_{r} - 1}} + {\sum\limits_{j = 0}^{N_{r} - 2}\;{{R_{i}(j)} \cdot 2^{j}}}}$

The (i mod 2)·2^(N) ^(r) ⁻¹ part of the above equation is represented inFIG. 5 by the toggle block T 218.

An address check is then performed on H(q) to verify that the generatedaddress is within the range of acceptable addresses: if (H(q)<N_(max)),where in one example N_(max) =6048 in the 8k mode, then the address isvalid. If the address is not valid, the control unit is informed and itwill try to generate a new H(q) by incrementing the index i.

The role of the toggle block is to make sure that we do not generate anaddress exceeding N_(max) twice in a row. In effect, if an exceedingvalue was generated, this means that the MSB (i.e. the toggle bit) ofthe address H(q) was one. So the next value generated will have a MSBset to zero, insuring to produce a valid address.

The following equations sum up the overall behaviour and help tounderstand the loop structure of this algorithm:

q = 0; for  (i = 0; i < M_(max); i = i + 1)$\{ {{{H(q)} = {{( {i\mspace{14mu}{mod}\mspace{11mu} 2} ) \cdot 2^{N_{r} - 1}} + {\sum\limits_{j = 0}^{N_{r} - 2}\;{{R_{i}(j)} \cdot 2^{j}}}}};\mspace{20mu}{{{if}\mspace{14mu}( {{H(q)} < N_{\max}} )\mspace{14mu} q} = {q + 1}};} \}$

As will be explained shortly, in one example of the address generator,the above mentioned permutation code is used for generating addressesfor all OFDM symbols. In another example, the permutation codes may bechanged between symbols, with the effect that a set of permutation codesare cycled through for successive OFMD symbols. To this end, the controllines 108, 110 providing an indication as to whether the OFDM symbol isodd or even and the current mode are used to select the permutationcode. This example mode in which a plurality of permutation codes arecycled through is particularly appropriate for the example in which theodd interleaver only is used, which will be explained later. A signalindicating that a different permutation code should be used is providedvia a control channel 111. In one example the possible permutation codesare pre-stored in the permutation code circuit 210. In another example,the control unit 224 supplies the new permutation code to be used for anOFDM symbol.

Receiver

FIG. 6 provides an example illustration of a receiver which may be usedwith the present technique. As shown in FIG. 6, a COFDM signal isreceived by an antenna 300 and detected by a tuner 302 and convertedinto a digital form by an analogue-to-digital converter 304. A guardinterval removal processor 306 removes the guard interval from areceived COFDM symbol, before the data is recovered from the COFDMsymbol using a Fast Fourier Transform (FFT) processor 308 in combinationwith a channel estimator and correction 310 in co-operation with aembedded-signalling decoding unit 311, in accordance with knowntechniques. The demodulated data is recovered from a mapper 312 and fedto a symbol de-interleaver 314, which operates to effect the reversemapping of the received data symbol to re-generate an output data streamwith the data de-interleaved.

The symbol de-interleaver 314 is formed from a data processing apparatusas shown in FIG. 7 with an interleaver memory 540 and an addressgenerator 542. The interleaver memory is as shown in FIG. 4 and operatesas already explained above to effect de-interleaving by utilising setsof addresses generated by the address generator 542. The addressgenerator 542 is formed as shown in FIG. 7 and is arranged to generatecorresponding addresses to map the data symbols recovered from eachCOFDM sub-carrier signals into an output data stream.

The remaining parts of the COFDM receiver shown in FIG. 6, including BitDe-Interleaver 316, are provided to effect error correction decoding 318to correct errors and recover an estimate of the source data.

One advantage provided by the present technique for both the receiverand the transmitter is that a symbol interleaver and a symbolde-interleaver operating in the receivers and transmitters can beswitched between the 1k, 2k, 4k, 8k, 16k and the 32k mode by changingthe generator polynomials and the permutation order. Hence the addressgenerator 542 shown in FIG. 7 includes an input 544, providing anindication of the mode as well as an input 546 indicating whether thereare odd/even COFDM symbols. A flexible implementation is therebyprovided because a symbol interleaver and de-interleaver can be formedas shown in FIGS. 3 and 7, with an address generator as illustrated inFIG. 5. The address generator can therefore be adapted to the differentmodes by changing to the generator polynomials and the permutationorders indicated for each of the modes. For example, this can beeffected using a software change. Alternatively, in other embodiments,an embedded signal indicating the mode of the DVB-T2 transmission can bedetected in the receiver in the embedded-signalling processing unit 311and used to configure automatically the symbol de-interleaver inaccordance with the detected mode.

Optimal Use of Odd Interleavers

As shown in FIG. 4, two symbol interleaving processes, one for evenCOFDM symbols and one for odd COFDM symbols allows the amount of memoryused during interleaving to be reduced. In the example shown in FIG. 4,the write in order for the odd symbol is the same as the read out orderfor the even symbol therefore, while an odd symbol is being read fromthe memory, an even symbol can be written to the location just readfrom; subsequently, when that even symbol is read from the memory, thefollowing odd symbol can be written to the location just read from.

The selection of the polynomial generator and the permutation codesexplained above have been identified following simulation analysis ofthe relative performance of the interleaver. The relative performance ofthe interleaver has been evaluated using a relative ability of theinterleaver to separate successive symbols or an “interleaving quality”.The relative measure of the interleaver quality is determined bydefining a distance D (in number of sub-carriers). A criterion C ischosen to identify a number of sub-carriers that are at distance≦D atthe output of the interleaver that were at distance≦D at the input ofthe interleaver, the number of sub-carriers for each distance D thenbeing weighted with respect to the relative distance. The criterion C isevaluated for both odd and even COFDM symbols. Minimising C produces asuperior quality interleaver.

$C = {{\sum\limits_{1}^{d = D}\;{{N_{even}(d)}/d}} + {\sum\limits_{1}^{d = D}\;{{N_{odd}(d)}/d}}}$

where: N_(even)(d) and N_(odd)(d) are number of sub-carriers in an evenand odd symbol respectively at the output of the interleaver that remainwithin d sub-carrier spacing of each other.

As mentioned above, during an experimental analysis of the performanceof the interleavers (using criterion C as defined above) and for exampleshown in FIG. 8( a) and FIG. 8( b) it has been discovered that theinterleaving schemes designed for the 2k and 8k symbol interleavers forDVB-T and the 4k symbol interleaver for DVB-H work better for oddsymbols than even symbols. Thus from performance evaluation results ofthe interleavers, for example for the 16k, as illustrated by FIGS. 8( a)and 8(b) it has been revealed that the odd interleavers work better thanthe even interleavers. This can be seen by comparing FIG. 8( a) whichshows results for an interleaver for even symbols and FIG. 8( b)illustrating results for odd symbols: it can be seen that the averagedistance at the interleaver output of sub-carriers that were adjacent atthe interleaver input is greater for an interleaver for odd symbols thanan interleaver for even symbols.

As will be understood, the amount of interleaver memory required toimplement a symbol interleaver is dependent on the number of datasymbols to be mapped onto the COFDM carrier symbols. Thus a 16k modesymbol interleaver requires half the memory required to implement a 32kmode symbol interleaver and similarly, the amount of memory required toimplement an 8k symbol interleaver is half that required to implement a16k interleaver. Therefore a transmitter or receiver which is arrangedto implement a symbol interleaver of a mode, which sets the maximumnumber of data symbols which can be carried per OFDM symbol, then thatreceiver or transmitter will include sufficient memory to implement twoodd interleaving processes for any other mode, which provides half orsmaller than half the number of sub-carriers per OFDM symbol in thatgiven maximum mode. For example a receiver or transmitter including a32k interleaver will have enough memory to accommodate two 16k oddinterleaving processes each with their own 16k memory.

Therefore, in order to exploit the better performance of the oddinterleaving process, a symbol interleaver capable of accommodatingmultiple modulation modes can be arranged so that only an odd symbolinterleaving process is used if in a mode which comprises half or lessthan half of the number of sub-carriers in a maximum mode, whichrepresents the maximum number of sub-carriers per OFDM symbol. Thismaximum mode therefore sets the maximum memory size. For example, in atransmitter/receiver capable of the 32k mode, when operating in a modewith fewer carriers (i.e. 16k, 8k, 4k or 1k) then rather than employingseparate odd and even symbol interleaving processes, two oddinterleavers could be used.

An illustration is shown in FIG. 9 of an adaptation of the symbolinterleaver 33, which is shown in FIG. 3 when interleaving input datasymbols onto the sub-carriers of OFDM symbols in the odd interleavingmode only. The symbol interleaver 33.1 corresponds exactly to the symbolinterleaver 33 as shown in FIG. 3, except that the address generator 102is adapted to perform the odd interleaving process only. For the exampleshown in FIG. 9, the symbol interleaver 33.1 is operating in a modewhere the number of data symbols which can be carried per OFDM symbol isless than half of the maximum number which can be carried by an OFDMsymbol in an operating mode with the largest number of sub-carriers perOFDM symbol. As such, the symbol interleaver 33.1 has been arranged topartition the interleaver memory 100. For the present illustration shownin FIG. 9 the interleaver memory then 100 is divided into two parts 401,402. As an illustration of the symbol interleaver 33.1 operating in amode in which data symbols are mapped onto the OFDM symbols using theodd interleaving process, FIG. 9 provides an expanded view of each halfof the interleaver memory 401, 402. The expanded view provides anillustration of the odd interleaving mode as represented for thetransmitter side for four symbols A, B, C, D reproduced from FIG. 4.Thus as shown in FIG. 9, for successive sets of first and second datasymbols, the data symbols are written into the interleaver memory 401,402 in a sequential order and read out in a permuted order in accordancewith the addresses generated by the address generator 102 as previouslyexplained. Thus as illustrated in FIG. 9, since an odd interleavingprocess is being performed for successive sets of first and second setsof data symbols, the interleaver memory must be partitioned into twoparts. Symbols from a first set of data symbols are written into a firsthalf of the interleaver memory 401, and symbols from a second set ofdata symbols are written into a second part of the interleaver memory402. This is because the symbol interleaver is no longer able to reusethe same parts of the symbol interleaver memory as can be accommodatedwhen operating in an odd and even mode of interleaving.

A corresponding example of the interleaver in the receiver, whichappears in FIG. 7 but adapted to operate with an odd interleavingprocess only, is shown in FIG. 10. As shown in FIG. 10 the interleavermemory 540 is divided into two halves 410, 412 and the address generator542 is adapted to write data symbols into the interleaver memory andread data symbols from the interleaver memory into respective parts ofthe memory 410, 412 for successive sets of data symbols to implement anodd interleaving process only. Therefore, in correspondence withrepresentation shown in FIG. 9, FIG. 10 shows the mapping of theinterleaving process which is performed at the receiver and illustratedin FIG. 4 as an expanded view operating for both the first and secondhalves of the interleaving memory 410, 412. Thus a first set of datasymbols are written into a first part of the interleaver memory 410 in apermuted order defined in accordance with the addresses generated by theaddress generator 542 as illustrated by the order of writing in the datasymbols which provides a write sequence of 1, 3, 0, 2. As illustratedthe data symbols are then read out of the first part of the interleavermemory 410 in a sequential order thus recovering the original sequenceA, B, C, D.

Correspondingly, a second subsequent set of data symbols which arerecovered from a successive OFDM symbol are written into the second halfof the interleaver memory 412 in accordance with the addresses generatedby the address generator 542 in a permuted order and read out into theoutput data stream in a sequential order.

In one example the addresses generated for a first set of data symbolsto write into the first half of the interleaver memory 410 can be reusedto write a second subsequent set of data symbols into the interleavermemory 412. Correspondingly, the transmitter may also reuse addressesgenerated for one half of the interleaver for a first set of datasymbols for reading out a second set of data symbols which have beenwritten into the second half of the memory in sequential order.

Odd Interleaver with Offset

The performance of an interleaver, which uses two odd interleavers couldbe further improved by using a sequence of odd only interleavers ratherthan a single odd only interleaver, so that any bit of data input to theinterleave does not always modulate the same carrier in the OFDM symbol.

A sequence of odd only interleavers could be realised by either:

-   -   adding an offset to the interleaver address modulo the number of        data carriers, or    -   using a sequence of permutations in the interleaver        Adding an Offset

Adding an offset to the interleaver address modulo the number of datacarriers effectively shifts and wraps-round the OFDM symbol so that anybit of data input to the interleaver does not always modulate the samecarrier in the OFDM symbol. Thus the address generator, could optionallyinclude an offset generator, which generates an offset in an addressgenerated by the address generator on the output channel H(q).

The offset would change each symbol. For example, this offset couldprovide be a cyclic sequence. This cyclic sequence could be, forexample, of length 4 and could consist of, for example, prime numbers.For example, such a sequence could be:

0, 41, 97, 157

Furthermore, the offset may be a random sequence, which may be generatedby another address generator from a similar OFDM symbol interleaver ormay be generated by some other means.

Using a Sequence of Permutations

As shown in FIG. 5, a control line 111 extends from the control unit ofthe address generator to the permutation circuit. As mentioned above, inone example the address generator can apply a different permutation codefrom a set of permutation codes for successive OFDM symbols. Using asequence of permutations in the interleaver address generator reduces alikelihood that any bit of data input to the interleaver does not alwaysmodulate the same sub-carrier in the OFDM symbol.

For example, this could be a cyclic sequence, so that a differentpermutation code in a set of permutation codes in a sequence is used forsuccessive OFDM symbols and then repeated. This cyclic sequence couldbe, for example, of length two or four. For the example of the 8k symbolinterleaver a sequence of two permutation codes which are cycled throughper OFDM symbol could be for example:

5 11 3 0 10 8 6 9 2 4 1 7 *

8 10 7 6 0 5 2 1 3 9 4 11

whereas a sequence of four permutation codes could be:

5 11 3 0 10 8 6 9 2 4 1 7 *

8 10 7 6 0 5 2 1 3 9 4 11

11 3 6 9 2 7 4 10 5 1 0 8

10 8 1 7 5 6 0 11 4 2 9 3

The switching of one permutation code to another could be effected inresponse to a change in the Odd/Even signal indicated on the controlchannel 108. In response the control unit 224 changes the permutationcode in the permutation code circuit 210 via the control line 111.

For the example of a 1k symbol interleaver, two permutation codes couldbe:

4 3 2 1 0 5 6 7 8

3 2 5 0 1 4 7 8 6

whereas four permutation codes could be:

4 3 2 10 5 6 7 8

3 2 5 0 1 4 7 8 6

7 5 3 8 2 6 1 4 0

1 6 8 2 5 3 4 0 7

Other combinations of sequences may be possible for 2k, 4k and 16kcarrier modes or indeed 0.5k carrier mode. For example, the followingpermutation codes for each of the 0.5k, 2k, 4k and 16k provide goodde-correlation of symbols and can be used cyclically to generate theoffset to the address generated by an address generator for each of therespective modes:

2k Mode:

0 7 5 1 8 2 6 9 3 4*

4 8 3 2 9 0 1 5 6 7

8 3 9 0 2 1 5 7 4 6

7 0 4 8 3 6 9 1 5 2

4k Mode:

7 10 5 8 1 2 4 9 0 3 6**

6 2 7 1 0 8 03 4 1 9 5

9 5 4 2 3 10 1 0 6 8 7

1 4 10 3 9 7 2 6 5 0 8

16k Mode:

8 4 3 2 0 11 1 5 12 10 6 7 9

7 9 5 3 11 1 4 0 2 12 10 8 6

6 11 7 5 2 3 0 1 1 0 8 12 9 4

5 12 9 0 3 10 2 4 6 7 8 11 1

For the permutation codes indicated above, the first two could be usedin a two sequence cycle, whereas all four could be used for a foursequence cycle. In addition, some further sequences of four permutationcodes, which are cycled through to provide the offset in an addressgenerator to produce a good de-correlation in the interleaved symbols(some are common to the above) are provided below:

0.5k Mode:

3 7 4 6 1 2 0 5

4 2 5 7 3 0 1 6

5 3 6 0 4 1 2 7

6 1 0 5 2 7 4 3

2k Mode:

0 7 5 1 8 2 6 9 3 4 *

3 2 7 0 1 5 8 4 9 6

4 8 3 2 9 0 1 5 6 7

7 3 9 5 2 1 0 6 4 8

4k Mode:

7 10 5 8 1 2 4 9 0 3 6 **

6 2 7 10 8 0 3 4 1 9 5

10 3 4 1 2 7 0 6 8 5 9

0 8 9 5 10 4 6 3 2 1 7

8k Mode:

5 11 3 0 10 8 6 9 2 4 1 7 *

10 8 5 4 2 9 1 0 6 7 3 11

11 6 9 8 4 7 2 1 0 10 5 3

8 3 11 7 9 1 5 6 4 0 2 10

*these are the permutations in the DVB-T standard

**these are the permutations in the DVB-H standard

Examples of address generators, and corresponding interleavers, for the2k, 4k and 8k modes are disclosed in European patent application number04251667.4, which corresponds to U.S. Pat. No. 8,155,228, the contentsof which are incorporated herein by reference. An address generator forthe 0.5k mode are disclosed in our co-pending UK patent applicationnumber 0722553.5. Various modifications may be made to the embodimentsdescribed above without departing from the scope of the presentinvention. In particular, the example representation of the generatorpolynomial and the permutation order which have been used to representaspects of the invention are not intended to be limiting and extend toequivalent forms of the generator polynomial and the permutation order.

As will be appreciated the transmitter and receiver shown in FIGS. 1 and6 respectively are provided as illustrations only and are not intendedto be limiting. For example, it will be appreciated that the position ofthe symbol interleaver and the de-interleaver with respect, for exampleto the bit interleaver and the mapper can be changed. As will beappreciated the effect of the interleaver and de-interleaver isun-changed by its relative position, although the interleaver may beinterleaving I/Q symbols instead of v-bit vectors. A correspondingchange may be made in the receiver. Accordingly the interleaver andde-interleaver may be operating on different data types, and may bepositioned differently to the position described in the exampleembodiments.

According to one implementation of a receiver, a data processingapparatus is provided to map symbols received from a predeterminednumber of sub-carrier signals of an Orthogonal Frequency DivisionMultiplexed (OFDM) symbol into an output symbol stream.

As explained above the permutation codes and generator polynomial of theinterleaver, which has been described with reference to animplementation of a particular mode, can equally be applied to othermodes, by changing the predetermined maximum allowed address inaccordance with the number of sub-carriers for that mode.

As mentioned above, embodiments of the present invention findapplication with DVB standards such as DVB-T, DVB-T2 and DVB-H, whichare incorporated herein by reference. For example embodiments of thepresent invention may be used in a transmitter or receiver operating inaccordance with the DVB-H standard, in hand-held mobile terminals. Themobile terminals may be integrated with mobile telephones (whethersecond, third or higher generation) or Personal Digital Assistants orTablet PCs for example. Such mobile terminals may be capable ofreceiving DVB-H or DVB-T/T2 compatible signals inside buildings or onthe move in for example cars or trains, even at high speeds. The mobileterminals may be, for example, powered by batteries, mains electricityor low voltage DC supply or powered from a car battery. Services thatmay be provided by DVB-H may include voice, messaging, internetbrowsing, radio, still and/or moving video images, television services,interactive services, video or near-video on demand and option. Theservices might operate in combination with one another. In otherexamples embodiments of the present invention finds application with theDVB-T2 standard as specified in accordance with ETSI standard EN 302755. In other examples embodiments of the present invention findapplication with the cable transmission standard known as DVB-C2.However, it will be appreciated that the present invention is notlimited to application with DVB and may be extended to other standardsfor transmission or reception, both fixed and mobile.

The invention claimed is:
 1. A data processing apparatus for mappingsymbols received from a predetermined number of sub-carrier signals ofan Orthogonal Frequency Division Multiplexed (OFDM) symbol into anoutput symbol stream, the data processing apparatus comprising: ade-interleaver configured to read-into a memory the predetermined numberof data symbols from the OFDM sub-carrier signals, and to read-out ofthe memory the data symbols into the output symbol stream to effect themapping, the read-out being in a different order than the read-in, theorder being determined from a set of addresses, with the effect that thedata symbols are de-interleaved from the OFDM sub-carrier signals, andan address generator configured to generate the set of addresses, anaddress being generated for each of the received data symbols formapping the data symbol received from the OFDM sub-carrier signal intothe output symbol stream, the address generator comprising a linearfeedback shift register including a predetermined number of registerstages and being configured to generate a pseudo-random bit sequence inaccordance with a generator polynomial, a permutation circuit configuredto receive the content of the shift register stages and to permute theorder of the bits present in the register stages in accordance with apermutation code to form an address of one of the OFDM sub-carriers, anda control unit configured in combination with an address check circuitto regenerate an address when a generated address exceeds apredetermined maximum valid address, wherein the permutation circuit isconfigured to change the permutation code, which permutes the order ofthe bits of the register stages to form the set of addresses from oneOFDM symbol to another.
 2. The data processing apparatus as claimed inclaim 1, wherein the permutation circuit is configured to cycle througha sequence of different permutation codes for successive OFDM symbols.3. The data processing apparatus as claimed in claim 1, wherein the OFDMsymbol includes pilot sub-carriers, which are configured to carry knownsymbols, and the predetermined maximum valid address depends on a numberof the pilot sub-carrier symbols present in the OFDM symbol.
 4. The dataprocessing apparatus as claimed in claim 1, wherein the number ofsub-carriers is determined by one of a plurality of operating modes, andwhen operating mode provides a number of the sub-carriers which is halfor less than half a maximum number of sub-carriers in the OFDM symbolsof any of the operating modes, and the data symbols include first setsof data symbols received from first OFDM symbols and second sets of datasymbols received from second OFDM symbols, the data processing apparatusis configured to de-interleave the first and second sets of data symbolsinto the output data stream in accordance with an odd interleavingprocess, the odd interleaving process including: writing the first setsof data symbols received from the sub-carriers of the first OFDM symbolsinto a first part of the interleaver memory in accordance with an orderdetermined by the set of addresses generated with one of the permutationcodes of the sequence, reading out the first sets of data symbols fromthe first part of the interleaver memory into the output data stream inaccordance with a sequential order of the first sets of input datasymbols, writing the second set of data symbols received from thesub-carriers of the second OFDM symbols into a second part of theinterleaver memory in accordance with an order defined by the set ofaddresses generated with another of the permutation codes of thesequence, and reading out the second sets of data symbols from thesecond part of the interleaver memory into the output data stream inaccordance with a sequential order of the second sets of input datasymbols.
 5. A receiver for receiving data from Orthogonal FrequencyDivision Multiplexing (OFDM) modulated symbols, the receiver beingconfigured to receive OFDM symbols, to recover the data symbols from apredetermined number of sub-carriers of the OFDM symbols, the receiverincluding a data processor which is configured to map the data symbolsreceived from the OFDM symbols into an output data stream, the dataprocessor comprising: a de-interleaver configured to read-into a memorythe predetermined number of data symbols from the OFDM sub-carriersymbols, and to read-out of the memory the data symbols into the outputsymbol stream to effect the mapping, the read-out being in a differentorder than the read-in, the order being determined from a set ofaddresses, with the effect that the data symbols are de-interleaved fromthe OFDM sub-carrier signals, and an address generator configured togenerate the set of addresses, an address being generated for each ofthe received data symbols for mapping the received data symbol from theOFDM sub-carrier signal into the output symbol stream, the addressgenerator comprising: a linear feedback shift register including apredetermined number of register stages and being configured to generatea pseudo-random bit sequence in accordance with a generator polynomial,a permutation circuit configured to receive the content of the shiftregister stages and to permute the order of the bits present in theregister stages in accordance with a permutation code to form an addressof one of the OFDM sub-carriers, and a control unit configured incombination with an address check circuit to re-generate an address whena generated address exceeds a predetermined maximum valid address,wherein the permutation circuit is configured to change the permutationcode, which permutes the order of the bits of the register stages toform the set of addresses from one OFDM symbol to another.
 6. Thereceiver as claimed in claim 5, wherein the receiver is configured toreceive data which has been modulated in accordance with a Digital VideoBroadcasting standard including the Digital VideoBroadcasting-Terrestrial standard, the Digital VideoBroadcasting-Handheld, or the Digital Video Broadcasting-Terrestrial2standard.
 7. A method of mapping symbols received from a predeterminednumber of sub-carrier signals of Orthogonal Frequency DivisionMultiplexed (OFDM) symbols into an output symbol stream, the methodcomprising: reading-into a memory the predetermined number of datasymbols from the OFDM sub-carrier signals, reading-out of the memory thedata symbols into the output symbol stream to effect the mapping, theread-out being in a different order than the read-in, the order beingdetermined from a set of addresses, with the effect that the datasymbols are de-interleaved from the OFDM sub-carrier signals, andgenerating the set of addresses, an address being generated for each ofthe received symbols for mapping the received data symbol from the OFDMsub-carrier signal into the output symbol stream, the generating the setof addresses comprising: using a linear feedback shift registerincluding a predetermined number of register stages to generate apseudo-random bit sequence in accordance with a generator polynomial,using a permutation circuit to receive the content of the shift registerstages and to permute the order of the bits present in the registerstages in accordance with a permutation code to form an address,re-generating an address when a generated address exceeds apredetermined maximum valid address, and changing, by the permutationcircuit, the permutation code, which permutes the order of the bits ofthe register stages to form the set of addresses from one OFDM symbol toanother.
 8. The method as claimed in claim 7, wherein the changing thepermutation code includes cycling through a sequence of differentpermutation codes for successive OFDM symbols.
 9. The method as claimedin claim 7, wherein the OFDM symbol includes pilot sub-carriers, whichare configured to carry known symbols, and the predetermined maximumvalid address depends on a number of the pilot sub-carrier symbolspresent in the OFDM symbol.
 10. The method as claimed in claim 7,wherein the number of sub-carriers is determined by one of a pluralityof operating modes, and when the and when operating mode provides anumber of sub-carriers which is half or less than half a maximum numberof sub-carriers in the OFDM symbols of any of the operating modes, andthe data symbols include first sets of data symbols received from firstOFDM symbols and second sets of data symbols received from second OFDMsymbols, and the reading-into the memory the predetermined number ofdata symbols from the OFDM sub-carrier signals, and the reading-out ofthe memory the data symbols into the output symbol stream is inaccordance with an odd interleaving process, the odd interleavingprocess including: writing the first sets of data symbols received fromthe sub-carriers of the first OFDM symbols into a first part of theinterleaver memory in accordance with an order determined by the set ofaddresses generated with one of the permutation codes of the sequence,reading out the first sets of data symbols from the first part of theinterleaver memory into the output data stream in accordance with asequential order of the first sets of input data symbols, writing thesecond set of data symbols received from the sub-carriers of the secondOFDM symbols into a second part of the interleaver memory in accordancewith an order defined by the set of addresses generated with another ofthe permutation codes of the sequence, and reading out the second setsof data symbols from the second part of the interleaver memory into theoutput data stream in accordance with a sequential order of the secondsets of input data symbols.